In recent years energy saving and miniaturization have been promoted in various fields. The fields of, for example, high-frequency radio communication devices represented by portable telephones and power supply apparatus are also influenced by them.
With a switching power supply apparatus driven at a high voltage, for example, a high breakdown voltage switching element with low loss whose switching frequency may be increased is proposed. Furthermore, with a high power amplifier for high-frequency radio communication the use of a transistor (amplifier element) which operates at high frequencies is also proposed in order to increase a maximum oscillation frequency. A transistor using a compound semiconductor, such as gallium nitride (GaN), whose on-state resistance is small or a silicon(Si) metal-oxide-semiconductor(MOS) transistor in which gate length is shortened is proposed as such a switching element or an amplifier element. Hereinafter a transistor using GaN and an Si-MOS transistor in which gate length is shortened will be referred to as a GaN transistor and a short-gate-length Si-MOS transistor respectively.
With a GaN transistor or a short-gate-length Si-MOS transistor, however, there is an upper limit to voltage applied to a gate portion which controls current (breakdown voltage of the gate portion). This upper limit is low compared with an ordinary MOS transistor. If voltage applied to the gate portion exceeds the upper limit, the gate portion may be destroyed.
In order to solve the problem of destruction of a circuit element by an overvoltage, formerly a Zener diode was connected to a terminal of a circuit element to be protected. If a voltage higher than or equal to a Zener voltage is applied to a Zener diode, a current flows through the Zener diode and a voltage higher than or equal to the Zener voltage is not applied to the terminal of the circuit element to be protected.
See, for example, Japanese Laid-open Patent Publication Nos. 09-69766 and 2008-113547.
However, if a Zener diode is used as a protective circuit for protecting a circuit element against an overvoltage, a power loss occurs because of a resistance component of the Zener diode when a current flows through the Zener diode.